A liquid crystal projection display apparatus using transparent type liquid crystal display elements have been noted in recent years as a substitute for projection type display apparatus using CRT. In particular, a liquid crystal color projection display apparatus using lights of three colors i.e., R, G and B, transmitting and modulating them separately into three liquid crystal display elements and then projecting them on a projection screen can provide color images over a large area and at high quality and, accordingly, vigorous development has now been undertaken for them.
Among such liquid crystal projection display apparatuses, those using TN (twisted nematic) type liquid crystal having advantageous feature of consuming less electric power and operating at low driving voltage are used predominantly.
FIG. 2 and FIG. 3 show examples of constitutions for liquid crystal color projection display apparatus using TN liquid crystal display elements or transparent-scattering type liquid crystal elements and dichroic mirrors.
FIG. 2 is an example of using dichroic prisms 12A, and 12B, each of which is a specific form of a dichroic mirror, for splitting and synthesizing color and they are so adapted that the incident angle .theta. of light is set as: .theta.=45.degree. in both of color splitting and color synthesis.
FIG. 3 shows another example of using dichroic mirrors 22A, 22B, 22C and 22D for color split and color synthesis, in which they are so adapted that the incident angle .theta. of light is set to: .theta.=45.degree. in both of color split and color synthesis.
In the figures, there are shown projection light sources 11 and 21, mirrors 13A, 13B, 13C, 13D, 23A and 23B, transparent-scattering type liquid crystal display elements (or TN liquid crystal display elements) 14A, 14B, 14C, 24A, 24B and 24C, optical projection systems 15 and 25 and projection screens 17 and 27.
Since two polarization plates are required in the TN type liquid crystal display element, it involves a problem that the light transmittance is low and the projected image is dark.
In particular, an extremely strong light source is required for projecting images and this leads to a problem that high contrast is difficult to be attained on the projection screen or heat generated from the light source gives an undesired effect on the liquid crystal display elements.
In view of the above, for overcoming the problems of the TN type liquid crystal display element, a transparent-scattering type liquid crystal display element using a liquid crystal and polymer composite has been proposed, in which a liquid crystal material is dispersed and held in a polymer matrix, the refractive index of the liquid crystal material changes depending on the state of voltage application to alternately provide a light transmission state and a light scattering state in accordance with agreement and disagreement of the refractive index between the liquid crystal material and the polymer matrix. The transparent-scattering type liquid crystal display element utilizes such light transparent-scattering properties.
In the transparent-scattering type liquid crystal display element using the liquid crystal and polymer composite based on the operation principle for the agreement of the refractive indices, two polarization plates are not required as in the TN type liquid crystal display element and the light emitted from the projection light source under random polarization can be used directly as the incident light. Accordingly, in the transparent-scattering type liquid crystal display element, a transmission image which is twice as bright as that of the TN type liquid crystal display element can be obtained even when an identical optical source is used.
However, if such transparent-scattering type liquid crystal display element is used, instead of the conventional TN type liquid crystal display element, to the liquid crystal color projection display apparatus as shown in FIGS. 2 and 3, it results in a drawback that the spectral characteristics are deteriorated in the optical system for color splitting or color synthesis, and the hue of the projected images is poor as compared with that of the liquid crystal color projection display apparatus using the TN type liquid crystal display element.
This is due to the use of dichroic mirrors or dichroic prisms for the color split means and the color synthesis means in the liquid crystal color projection display apparatus. That is, when white light emitted from a light source of white color is passed through color split means comprising dichroic mirrors or dichroic prisms, a plurality of split lights of different colors can be obtained. On the other hand, when a plurality of thus split lights are passed through the color synthesis means comprising the dichroic mirror or dichroic prism, lights synthesized from them can be obtained.
The dichroic mirror or dichroic prism has a structure in which transparent dielectric films each of different refractive index are laminated, each at a film thickness substantially corresponding to the light wavelength, on a transparent sheet or the surface of a prism, and it has a function of spectralizing light into that in a high transmission wavelength region and that in a high reflection wavelength region with respect to an optical wavelength depending on the structure of the multi-layered film by the optical multi-interference effect. It has been known that such an optical multi-layered film shows remarkable difference in the spectral characteristics corresponding to P polarization and S polarization relative to the surface formed with the multi-layered films along with the increase of the light incident angle .theta. from zero. FIGS. 5 and 6 show one example of the dependence of the spectral characteristics on the state of polarization of the dichroic mirror and the dichroic prism.
In the case of the liquid crystal color projection display apparatus using the TN type liquid crystal display element, since the polarization plates are used, only one of P polarization or S polarization is utilized by disposing the plates such that the polarization axis corresponds to only one of the polarizations. Accordingly, even if the polarization dependency of the spectral characteristics is formed as shown in FIGS. 5 and 6, sharp color splitting characteristics can be obtained and a projected image with satisfactory hue can be obtained.
On the other hand, since incident light is put under random polarization in the case of using the transparent-scattering type liquid crystal display element, the dichroic mirror or the dichroic prism shows a spectralizing effect corresponding to the mean value between the P polarization and the S polarization in the spectral characteristics. Accordingly, since the color splitting characteristics are as shown by dotted lines in FIGS. 5 and 6, in which the color purity is reduced. Therefore, the hue of the projection image after color synthesis is poor as compared with that in the liquid crystal color projection display apparatus using the TN type liquid crystal display element.